The invention further concerns an implantable electrostimulation device for the treatment of tachycardiac rhythm disturbances, comprising a detection unit, a control unit and a therapy unit which is adapted to produce a cardioversion or defibrillation electrotherapy to be transmitted to the heart, wherein the detection unit has a measuring unit for picking up and reproducing a measurement signal corresponding to intracardial impedance at its output and an evaluation unit connected at the input to the measuring unit and the control unit receives output signals from the evaluation unit and controls the activity of the measuring unit, the evaluation unit and the therapy unit.
By virtue of the lower specific resistance of blood in comparison with the myocardium tissue intracardial impedance varies with the volume of blood in the chambers of the heart, which changes in the course of the cardiac cycle. Thus, the pump activity of the heart can be monitored by evaluation of an intracardial impedance measurement and the existence of cardiac rhythm disturbances such as tachycardia or fibrillation can be inferred from variations in the amplitude pattern or the frequency of the periodic impedance signal.
Such a monitoring device is known from European patent application No. 0 009 255 A1, to Geddes, published 2 Apr. 1980. That publication discloses an implantable defibrillator having two measuring electrodes arranged at an axial spacing from each other at the distal end of a catheter which is introduced into the right ventricle. A control logic unit starts an intracardial impedance measurement procedure when automatic evaluation of an ECG signal indicates the possible existence of fibrillation. Impedance is determined by means of voltage measurement between the electrodes with an alternating current flow with a constant modulation amplitude and subsequent demodulation of the voltage signal. If the amplitude of the impedance signal indicates excessively low pump activity on the part of the heart defibrillation therapy is initiated.
A disadvantage with that device is that the signals from the measuring electrodes react sensitively to interference effects by virtue of their arrangement in a ventricle. Thus, body movement can already give rise to contact between the electrodes and the myocardium tissue. That means however that the measurement voltage and thus impedance measurement are falsified. Unnecessary defibrillation shocks which are painful to the patient can be the consequence of defective impedance measurement.
It is known from U.S. Pat. No. 5,427,112, to Noren, issued 27 Jun. 1995, for increasing the reliability of detection of cardiac rhythm disturbances, to record two signals and to monitor the in-phase periodic recurrence thereof, which is coupled to the heartbeat. In addition to measurement of the intracardial impedance signal, that known device also provides for determining the derivative thereof in respect of time. The impedance signal is recorded in a parameter representation as a function of its derivative in respect of time. The curve recorded in that way is compared to stored pattern curves, whereupon a decision is made about the necessity for and possibly the nature of a therapy. That procedure suffers from the disadvantage that it is highly costly in terms of computation and memory; it is firstly necessary to determine the derivative in respect of time of the measurement signal. The measurement value together with the derivative in respect of time have to be stored over at least the duration of a cardiac period. Then the phase position of the stored data has to be determined, in comparison with a previously stored pattern curve, and that requires extensive computations. Finally it is then necessary to form the difference of the measured pairs of values and corresponding pairs of values of the pattern curve, and ultimately evaluate same on the basis of a mathematical criterion.
U.S. Pat. No. 5,179,946, to Weiss, issued 19 Jan. 1993, discloses an implantable defibrillator in which, to increase the level of reliability of detection of cardiac rhythm disturbances, the intracardial impedance between two defibrillation electrodes fixed to the outside of the heart is measured. Adequate pump activity on the part of the heart is monitored on the basis of the impedance signal, by means of an amplitude discriminator. As an alternative to amplitude discrimination that known defibrillator provides for integration of the measured impedance signal. That device is complicated and expensive in circuitry terms because impedance measurement and tachycardia therapy are effected by way of the same electrodes. Therefore, avoiding destruction of the measuring unit by the high electrical voltages which are produced in a cardioversion or defibrillation procedure requires a protective circuit which is to be connected between the defibrillation electrodes and the measuring unit prior to application of the therapy. A disadvantage with that known defibrillator is also major operative involvement which stresses the patient when implanting the epicardial electrodes.